Microstrip patch antennas of a proposed type would be tunable over broad wavelength ranges. These antennas would be attractive for use in a variety of microwave communication systems in which there are requirements for transmission and/or reception at multiple, widely separated frequencies.

efforts have been somewhat successful, but have yielded tuning ranges of 20 percent and smaller — much smaller than needed in typical practical cases.

Like prior microstrip patch antennas (both tunable and non-tunable), the proposed antennas would have instantaneous bandwidths of about 1 percent of their nominal or resonance frequencies. However, these would be tunable over much broader frequency ranges — as much as several octaves, depending on specific designs. They could be fabricated relatively simply and inexpensively by use of conventional photolithography, and without need for integration with solid-state electronic or piezoelectric control devices.

An antenna as proposed (see figure) would include a microstrip patch radiating element on a thin ferroelectric film on a semiconductor substrate with a ground-plane conductor on the underside of the substrate. The ferroelectric film could be, for example, SrTiO3 with a thickness of the order of 1 or 2 µm.

To enable operation at multiple desired frequencies, the antenna would be designed to resonate in a fundamental TM01 mode that lies at an odd common denominator of the desired frequencies. (In an odd mode, the antenna would radiate strongly in a direction perpendicular to the antenna plane, while an even mode would place a null of the radiation pattern in the perpendicular direction.)

A simple cavity mathematical model yields the following equation for the resonance frequency of the m,n mode:

fmn = (c/2Πer1/2)[(m/w)2+(n/l)2]1/2,

where m and n are integers, c is the speed of light in vacuum, er is the effective relative permittivity of the combination of the ferroelectric film and dielectric substrate, and w and l are the width and length of the patch, respectively. In practice, resonance frequencies tend to be somewhat lower than predicted by this simple formula, because of fringing fields.

A Multimode Patch Antenna would be designed to resonate at an odd submultiple of possibly widely separated desired operating frequencies. By use of a combination of (1) varying the DC bias voltage to vary the permittivity of the ferroelectric film and (2) selection of the desired harmonic frequency, one could effectively achieve tuning over an unprecedentedly wide frequency range.
The value of er, and hence the resonance frequency, could be controlled by applying an electric field across the ferroelectric film. For this purpose, a controllable DC bias potential would be applied between the microstrip patch and the ground plane. A virtual ground plane can also be manipulated in this manner by controlling the carrier population in the semiconductor. Positive bias on the patch forms a sea of electrons near the ferroelectric interface, whereas a reverse bias forms a depletion layer. For isolation of the DC bias source from the radio-frequency signal, the DC bias would be fed in via a high-impedance microstrip transmission line with one end connected to a corner of the patch and the other end terminated in a quarter-wave radial stub (the quarter-wavelength radius would be chosen for a frequency near the middle of the desired radiation-frequency range). A wire would deliver the bias voltage to a radio-frequency virtual-short-circuit location on the transmission line so that the impedance would not be perturbed.

This work was done by Robert R. Romanofsky of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Computers/Electronics category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4—8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16792.


NASA Tech Briefs Magazine

This article first appeared in the September, 2003 issue of NASA Tech Briefs Magazine.

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